The present invention relates to a wireless repeater system which amplifies a receive signal and transmits the amplified signal using the same transmit frequency as the receive frequency.
FIG. 1 shows general concept of a prior repeater system in which the transmit frequency is the same as the received frequency. In the figure, the repeater has a receiver antenna 1 with the gain G.sub.1, the amplifier 2 with the gain G.sub.2, and the transmitter antenna 3 with the gain G.sub.3. The received signal 4 is amplified by the amplifier 2, which provides the transmitted signal 5. The frequency of the received signal 4 is the same as that of the transmitted signal 5, and the total gain in the repeater is G.sub.1 .times.G.sub.2 .times.G.sub.3. The numeral 6 shows the spillover signal of the transmitted signal at the receiver antenna 1. When the transmission loss between the transmitter antenna and the receiver antenna is L.sub.5, the level of the signal 6 is 1/L.sub.5 as high as the signal 5.
When the transmission loss L.sub.5 between the antennas is less than the total gain of the repeater (L.sub.5 &lt;G.sub.1 .times.G.sub.2 .times.G.sub.3), the system is unstable, and will oscillate and disturb communication. Therefore, the directivity of the antennas and the amplifier of the repeater are designed so that the transmission loss L.sub.5 between the antennas is considerably larger than the total repeater gain.
FIG. 2 show a block diagram of a prior repeater which measures the transmission loss between the antennas. In the figure, the test signal with the level A.sub.1 and the frequency f.sub.1 from the signal generator 7 is applied to the amplifier 2 which has the gain G.sub.2, so that the signal with the frequency f.sub.1 is transmitted in the air through the antenna 3 which has the gain G.sub.3. The signal level detector 8 measures the spillover signal 6 through the receiver antenna 1 which has the gain G.sub.1. When the measured value is A.sub.0, the transmission loss L.sub.5 is; EQU L.sub.5 =(A.sub.1 /A.sub.0).times.G.sub.1 .times.G.sub.2 .times.G.sub.3
Therefore, the value G.sub.2 is designed so that L.sub.5 &gt;G.sub.1 .times.G.sub.2 .times.G.sub.3 is satisfied.
However, one disadvantage is that this measurement is not real-time. Therefore, even when the transmission loss is large when the system is installed or designed, the transmission loss will be decreased by any change in the surroundings of the system. For instance, when a reflection body for an electric wave is built close to the antennas, the transmission loss between the antennas would be decreased as compared with that of the initial value.
Another prior art for measuring transmission loss is shown in the Japanese patent laid open publication No. 29921/74, in which the transmit signal is AM modulated by a pilot signal, which is detected by the receiver side. In this case, the desired received signal and the undesired interference signal (spillover signal) are combined. Since the phase of the interference signal does not coincide with the desired received signal, the detected level by an envelope detector has frequency characteristics. Therefore, the accurate measurement of the spillover signal is impossible using a simple method as an envelope detector method.
Still another prior art for measuring transmission loss of a spillover signal is shown in the Japanese patent laid open publication No. 95435/83, in which a pilot signal is transmitted, and the received level of that pilot signal is measured. However, the disadvantages are that (a) must have an additional band for transmitting a pilot signal, and (b) only the transmission loss at the pilot frequency is measured, but the transmission loss at other frequencies can not be measured.
Still another prior art for measuring transmission loss of a spillover signal is shown in U.S. Pat. No. 4,475,243, in which a time delay of a spillover signal is measured and the correlation between a transmit signal and a received signal is measured in order to compensate the spillover signal. However, one disadvantage is that accurate measurement is impossible when the time delay is small as is the case of a booster for a mobile communication.
Still another prior art for measuring transmission loss of a spillover signal is shown in the Japanese patent laid open publication No. 66025/79, in which the output power of the transmitter is measured. When the system oscillates, the transmission power increases, thus, the oscillation of the system is detected by measuring the transmission power in order to stop working or reduce its amplification. However, this technique has the disadvantage that the abnormal condition is detected only by the oscillation, and the system does not recover even after the possibility of oscillation disappears.